In a superhet receiver, a received RF signal is first transformed from a linear or circular polarised wave (as transmitted) to a guided wave or a travelling wave in a suitable transmission medium by the use of an antenna. The travelling wave is then applied to a mixer circuit where it is mixed with a local oscillator signal to form an intermediate frequency (IF) signal. The IF frequency may equal the simple difference between the RF and local oscillator frequencies or the difference between the RF frequency and a harmonic of the local oscillator frequency, which being dependent upon whether a `fundamental` or a `harmonic` mixer is used.
In a conventional receiver the antenna and mixer are two separate entities coupled via a transmission medium but it has been proposed, in the interests of improved ruggedness, simplification and reduced cost, to integrate the mixer into the antenna structure. In this proposal, various fundamental mixers, e.g. balanced and dual balanced, are suggested. Essentially, however, each comprises a pair of crossed dipoles mounted on, or in very close proximity to, a high dielectric support body, with a mixer diode ring connected between the constituent limbs of the dipoles. The RF and local oscillator signals with respective linear polarisations orthogonal one to another are radiatively coupled to respective ones of the dipoles. It has also been proposed to make a harmonic mixer in which a planar dipole is mounted upon a dielectric body with at least one diode connecting the dipole limbs and in which the local oscillator signal is `directly injected` into the mixer, i.e. in which the local oscillator signal is fed to the mixer via a particular form of conductive link instead of being radiated onto the mixer.